Powder coating compositions almost always contain a certain amount of pigments to reach the aesthetic requirements. As known in the field of powder coating it is impossible to include high levels of pigments in the coating composition. A high level of pigment always gives rise to an unacceptable appearance of the final, cured, coating. For example flow, gloss, and haze deteriorate. Therefore it has never been possible to include large amounts of pigment into the coating composition. With high level is meant here levels of approximately more than 40-50 w/w % (amount of pigment in the coating composition).
In U.S. Pat. No. 6,669,984 a powder coating composition is described that comprises a powder-based binder, a pigment and a surface-active agent. The binder consists of the combination of a resin and a cross-linking agent. U.S. Pat. No. 6,669,984 seeks to improve the color strength without sacrificing too much on other physical properties. In a mixing and extrusion step the binder is combined with a pigment and a surface-active agent. By applying this procedure, U.S. Pat. No. 6,669,984 is able to incorporate up to 50% pigment in the coating mixture consisting of the binder, the pigment and the surface-active agent.
A disadvantage of U.S. Pat. No. 6,669,984 is that with higher levels of pigment the final coating has an unacceptable appearance. However as it is sometimes necessary or desirable to incorporate more than 50% pigment in the coating composition without deteriorating the haze, a solution to this problem had to be found. An example of the necessity of a higher than 50% pigment-loading is when only thin layers of the coating composition are applied, whereby it is still necessary to reach the desired and specified requirements such as for example high “hiding power”.
It is an object of the present invention to overcome this disadvantage. This is reached by a composition suitable for a powder coating composition comprising at least one resin and at least one dispersant whereby the dispersant is added during the resin synthesis or while the resin leaves the reactor. With reactor is here and hereinafter meant the reactor wherein the resin is synthesized. It has surprisingly been found that a powder coating composition that comprises the composition according to the present invention shows no significant increase in the amount of haze when more than 50 w/w % pigment is added.
An additional advantage of the composition according to the invention is that the flow of a powder coating composition comprising the composition according to the invention is generally better than a powder coating composition known in the prior art. Further the gloss and processability of the powder coating composition are improved when the composition according to the invention is used.